The technical field of this invention is surgical instruments and procedures and, in particular, systems and methods for inhibiting restenosis associated with angioplasty.
Atherosclerosis is a disease which causes thickening and hardening of the arteries, characterized by lesions of raised fibrous plaque formed within the arterial lumen. Atherosclerotic plaque is commonly treated by means of angioplasty through the use of a balloon catheter. Balloon angioplasty involves passing a small, balloon-tipped catheter percutaneously into an artery and up to the region of obstruction. The balloon is then inflated to dilate the area of obstruction. Other devices, such as atherosclerectomy instruments which remove obstructions by dealing or shaving plaque from the artery wall, are also utilized in the treatment of atherosclerosis. More recently, laser systems have been proposed for performing angioplasty. In laser angioplasty, a catheter carrying a fiber optic waveguide is passed through a blood vessel, positioned near an obstruction, and then activated to decompose the plaque with laser radiation.
At present, over 200,000 angioplasty procedures are performed each year in the United States. Unfortunately, restenosis, or closure of the blood vessel following angioplasty, is a common occurrence following all types of such surgery. Approximately 30% of segments dilated by means of balloon catheter will develop significant restenosis, with peak incidence occurring between 2 and 3 months after angioplasty. Similar restenosis rates accompany laser angioplasty. When restenosis occurs, further coronary difficulties can result including strokes, arrhythmia, infarcts and even death.
Evidence suggests that intimal hyperplasia or proliferation of smooth muscle cells is a major factor in restenosis. proliferation of smooth muscle cells is very common in patients after angioplasty, whether or not restenosis occurs. Medial smooth muscle cells, a main component of the arterial wall, proliferate in response to any injury to the arterial wall. Smooth muscle cells enter their growth cycle between 2 and 3 days after injury, and the majority of smooth muscle cells will cease to proliferate within 7 days. The total number of smooth muscle cells in the intima reaches a peak about two weeks after injury and remains constant for up to one year, suggesting that a reduction of the number of smooth muscle cells injured during angioplasty will reduce the likelihood of subsequent restenosis. See, generally, Liu et al., "Restenosis After Coronary Angioplasty, Potential Biologic Determinants and Role of Intimal Hyperplasia," Vol. 79, Circulation, pp. 1374-1387 (1989).
At present, efforts to prevent restenosis typically consists of drug therapy or modification of angioplasty techniques. Drug therapy is primarily directed toward the control of restenosis through the use of antiplatelet agents, antiproliferative agents, or antimigratory agents. The goal of drug therapY is to reduce smooth muscle cell proliferation by attacking the smooth muscle cells directly, or by affecting processes that promote smooth muscle cell proliferation. Unfortunately, most of the drugs under investigation are unproven, with unknown efficiency and side effects.
An alternative approach to reduce restenosis is to modify the techniques used in performing angioplasty. Until recently, angioplasty was performed by passing a small, balloon-tipped catheter percutaneously to an obstruction site and then inflating the balloon to dilate the area of obstruction. In balloon angioplasty, the outward compression of the balloon stresses the vessel walls, often resulting in cracking or tearing of the wall and injury to the smooth muscle cells. This injury, in turn, increases the risk of restenosis. One method to reduce restenosis resulting from balloon angioplasty is to heat the balloon during dilation to "seal" the injured vessel wall. See, for example, U.S. Pat. No. 4,754,752 issued to Ginsberg et al. on July 5, 1988.
Modified forms of laser angioplasty have also been proposed to remove atherosclerotic obstructions. Up until recently, researchers in laser angioplasty primarily have relied upon continuous wave (CW) lasers. Such lasers, while sufficient to ablate an obstruction, can also substantially cause thermal injury to vessel walls adjacent to the obstruction. Recently, high energy excimer lasers and other pulsed laser sources, which possess high peak intensity levels and very rapid pulse rates, have been found to destroy the target obstruction while minimizing the thermal injury to surrounding tissue.
Nonetheless, even with these less traumatic procedures, restenosis continues to be a significant factor compromising the effectiveness of angioplasty.
There exists a need for better methods and devices for preventing restenosis after angioplasty. A system which could perform angioplasty, while reducing the likelihood of smooth muscle cell proliferation in the vicinity of the angioplasty site, would satisfy a significant need in the art.